1. Field of the Invention
The invention relates to the field of photolithographic fabrication processes for microelectronics fabrication. More particularly the invention relates to photomasks employed in photolithographic pattern generation for microelectronics fabrication.
2. Description of the Related Art
Microelectronics fabrications are constructed of multiple layers of microelectrornics materials formed in laminar fashion sequentially on a substrate or carrier. Many of the microelectronics layers are patterned, and the patterns must be matched or registered to each other with extreme precision and accuracy. A common method for forming a patterned microelectronics layer employs a patterned etch mask layer formed over the microelectronics layer followed by subtractive etching of the microelectronics layer. Alternatively, the patterned mask layer may be used to form a second microelectronics layer over the first by an additive process.
A patterned etch mask layer usually consists of an organic polymer light sensitive layer known as a photoresist layer which has been exposed to the chosen pattern through a corresponding exposure mask and the resulting latent pattern subsequently developed. The light shining through the transparent portions of the exposure mask causes a chemical change in the photoresist layer material thus exposed compared to the portions which were not exposed due to the opacity of the exposure pattern, and hence appropriate chemical treatment differentiates between exposed and unexposed portions of the photoresist, allowing the exposure pattern to be replicated in the photoresist layer. The photoresist pattern may be a direct copy of the exposure pattern if the photoresist is a positive-working resist, or the photoresist pattern may be a reversed or negative copy of the exposure pattern if the photoresist is a negative-working photoresist.
The exposure of the photoresist may be accomplished directly from a contact or close proximity alignment of the exposure pattern (contact mask), followed by illumination with a collimated source of light, or by projection of the optically reduced image of the exposure pattern (reticle) on to the surface of the photoresist layer. In either case, the boundary between a clear region transmitting light and an opaque region blocking light in the original image of the pattern is ideally as sharp as possible. In practice, the sharpness of the transition from clarity to opacity is limited by several factors. The ultimate limit is set by diffraction of the exposing light at the boundary edge, which is greater for longer wavelengths. For this reason the exposure conditions have proceeded to ever shorter wavelength radiation, so that currently there is wide use of deep ultraviolet (DUV) radiation for photoexposure of patterns.
Other limits are set by available materials and methods employed. For masking off illumination, it is necessary to use highly opaque materials such as chromium in the form of extremely thin film layers on optically clear substrates of high transmittance, such as optical grade quartz. Light scattering at the edge of even the sharpest image boundary may lead to a less than abrupt edge in the corresponding photoresist pattern. To minimize this effect, a phase shift photomask pattern may be employed. In this type of photomask, there is formed an image pattern which combines opacity and transmittance properties of a material formed into a patterned phase shift layer on a substrate material. The phase shift material is translucent to the exposing radiation of wavelength L and possesses a refractive index n.sub.i such that if, after being formed as a layer of thickness x, there is satisfied the relationship: ##EQU1##
then there is a destructive interference of the light being transmitted by the clear region and the light partially transmitted adjacently by the translucent image phase shift layer. This effect pertains whenever the thickness difference between the substrate surface and the phase shift layer is a half-integral multiple of L.
Phase shift photomasks for use with deep ultraviolet (DUV) exposure radiation are generally formed employing quartz substrates with high DUV transmittance for the clear areas and a phase shift DUV absorbing patterned layer formed of an etchable translucent material such as spin-on-glass (SOG) dielectric material, for example. This combination is known as a single half-tone phase shift mask, and has the advantage of both clear and absorbing areas being transparent to visible light which is usefull for inspection purposes An optional thin partially absorbing layer such as chromium may be employed in a coincident pattern fashion with the phase shift layer, in which case the mask is a multiple layer half tone phase shift mask. In many cases a pattern is engraved in the quartz layer adjacent to the patterned phase shift layer so that the thickness of the phase shift layer and the depth of the adjacent quartz engraved pattern are equal to the appropriate half-integral multiple of the wavelength L to embody the destructive phase shift.
Although various forms of phase shift mask are satisfactorily employed for DUV exposure in microelectronics fabrication, such phase shift masks are not without problems. For example, the control of the phase shift is critical, and phase shift masks which are only a few degrees away from the ideal phase shift of 180.degree. may be unsuitable for use and rejected on inspection. Furthermore, the degree to which transmittance and phase shift can be manipulated to attain optimum mask performance may require incompatible methods of modulating either of these properties for a given mask material set. Generally a change in phase angle caused by etching to decrease thickness can cause a thickness change and hence a phase angle change incrementally in one direction only, i. e. in whatever direction a subtractive change provides.
It is thus towards the goal of providing methods for incremental modification of the phase angle of a phase shift mask to attain the specified values for both phase angle and transmittance that the present invention is generally and specifically directed.
Various methods have been disclosed for forming and modifying phase shift masks employed in microelectronics fabrication.
For instance, Tarumoto et al., in U.S. Pat. No. 5,702,847, disclose a method for forming a phase shift mask with little or no distortion or defects from the peripheral region. The method employs a spin-on-glass (SOG) dielectric material as the phase shift layer, and selectively removes by a lift-off method the thicker resion of the SOG which tends to form at the periphery during spin application.
Further, Yokoyama et al., in U.S. Pat. No. 5,723,234, disclose a method for dry etching a phase shift photomask to produce a uniform pattern dimension even where there is a large difference in exposed area ratio for different portions of the mask The method provides a dummy pattern in an unused portion of the mask area for dry etch correction of the image pattern.
Still further, Mlitsui, in U.S. Pat. No. 5,804,337, discloses a method for producing a phase shift mask with excellent contrast at the boundary of the pattern. The method provides a translucent phase shift layer deposited on a blank transmitting substrate employing sputtering of a mixture of a metal, silicon, oxygen and nitrogen.
Finally, Lee, in U.S. Pat. No. 5,851,705, discloses a method for forming a self-aligned phase shift mask. The method employs light-shielding patterns formed on a substrate over which a photoresist layer is formed. Infusing the photosensitive material with an organic material with an alkaline component is followed by back exposure and development to produce an overhanging structure which acts as a mask for selective etching of the phase shift pattern into the substrate.
Desirable in the art of microelectronics fabrication are further methods for fabricating a phase shift photomask with the capability of modulation of phase angle without affecting transmittance.
It is towards this goal that the present invention is generally directed.